1. Field of the Invention
The present invention relates generally to energy transfer elements and, more specifically, the present invention relates to energy transfer elements having at least 3 windings and electrically conductive shields.
2. Background Information
FIG. 1 shows an outline schematic diagram of a flyback converter power supply 101. The basic operation of the flyback converter 101 power supply is well documented and known to one skilled in the art. The primary (or input) circuit comprising primary switch 103 is controlled through a feedback control signal 105, typically but not necessarily from the secondary (or output) circuit of the power supply as shown. The energy transfer element, or transformer 107, which for the purpose of this description is shown having only 2 windings, uses a winding dot polarity that indicates the phase relationship of the winding voltages. During voltage transitions across the windings, the dot end of the windings are in phase.
FIG. 2 is a schematic of a power supply 201, which expands on the outline schematic of FIG. 1 by representing the parasitic capacitances 209 that exist between the transformer body, core or structure (energy transfer element) and electrical earth, the parasitic capacitances 211 that exist between the input and output windings and the transformer body (core) and also the parasitic capacitances 213 that exist between the input and output windings of the transformer. Usually the transformer core is the ferrite core used in the transformer construction to provide a low reluctance path for the magnetic flux coupling input and output windings of the transformer 207. As noted in FIG. 2, the parasitic capacitance 215 between the output of the transformer and electrical earth in some embodiments maybe short circuited depending on the application and or the way in which the electrical noise measurements are made.
During the normal operation of the power supply 201, the voltages across both input and output windings of the transformer 207 transition in accordance with the standard flyback converter power supply operation. These transitions generate displacement currents in the various parasitic capacitances 209, 211, 213 and 215 shown. These displacement currents are detected as common mode noise (or emissions) and measured by a piece of test equipment called a Line Input Stabilization Network (LISN). The configuration and connection of this equipment is well documented and known to one skilled in the art.
FIG. 2 also highlights capacitor Cy 217, which is a Y-capacitor that is commonly used in switching power supplies to reduce the common mode emissions. This component, capacitor Cy 217, provides a low impedance path for displacement currents flowing between input and output circuits of the flyback converter 201, to return to their source without flowing through electrical earth. The currents in capacitor Cy 217 are not detected by the LISN and its use therefore acts to reduce common mode emissions.